If one studies on a full-time basis using only the UK QDB (Question Data Base) like Bristol, CATS etc. (around 9000 questions, compare this to Aviationexam.com ~ 18.000) you can probably do it in about 10 month or so. BUT, if you have to wok a full-time job besides studying, especially a job including three shifts, I'd say expect at least 18 month plus.